1. Field of the Invention
This invention relates to an electrostatic chuck which is useful in electrostatically holding a substrate in position in the course of the fabrication of semiconductor integrated circuits and, particularly, in the step of plasma etching.
2. Description of the Prior Art
In the plasma etching step in the manufacture of semiconductor integrated circuits, it is usual to employ a wafer chuck of an electrostatic attachment type or the Jansen-Rerbeck type, or a so-called electrostatic chuck. The chuck essentially includes an insulating layer. As such an insulating layer, there has been proposed the use of plastics such as polyimides, ceramics such as alumina, aluminum nitride and the like, and rubber elastomers such as silicone rubbers. These materials have now been in use.
In the plasma etching step, it is essential that a wafer be uniformly maintained at a given temperature while suppressing the temperature rise of the wafer as will be caused by the heat generated from a plasma, thus ensuring high precision etching. For this purpose, a cooling means for passing a cooling medium in contact with one side of the chuck which is opposite to the wafer side is provided. The wafer can be cooled and uniformly maintained at a given temperature.
With the electrostatic chuck having a ceramic insulating layer, the insulating layer exhibits good durability against a plasma gas and has high thermal conductivity. This type of chuck has now been put into practice. However, the insulating layer which is placed in face-to-face relation with the wafer is so hard that its contact with the wafer is not so intimate. Eventually, the contact heat resistance becomes great, with the attendant problem that a satisfactory heat dissipation or cooling characteristic cannot be expected. In order to solve this problem, the usual practice is to pass an inert gas such as helium between the wafer and the insulating layer, thereby permitting the thermal transfer to smoothly take place between the wafer and the insulating layer through the inert gas. However, the passage of the inert gas requires fine processing of the insulating layer to form grooves in the layer, resulting in the high fabrication cost of the chuck. Moreover, a device for passing the inert gas is necessary, inviting an additional cost.
The electrostatic chuck using a plastic insulating layer such as a polyimide layer is not always satisfactory with respect to the durability against a plasma gas, but is easy in its manufacture and inexpensive. Thus, such a chuck has currently been in wide use. However, the plastic material is low in thermal conductivity and hard, so that like the ceramic electrostatic chuck, the plastic chuck is unlikely to intimately contact with a wafer. Because of the great contact heat resistance, a good cooling characteristic cannot be expected as well.
Japanese Laid-open Patent 59-64245 proposes an electrostatic chuck using a silicon rubber insulating layer. More particularly, the electrostatic chuck includes a metallic plate, a first insulating film composed of a heat-dissipating silicone prepreg impregnated in a glass cloth, a copper pattern formed on the first insulating film and serving as electrodes, and a second insulating film formed on the copper pattern and composed of a silicone rubber. Since the second insulating layer consists of the elastic silicone rubber, the contact heat resistance is relatively small. Thus, the layer exhibits a good heat-dissipating characteristic. A wafer which is brought into contact with the second insulating layer can be readily, uniformly maintained at a given temperature in an efficient manner.
In recent years, however, there is a demand for an electrostatic chuck which has the capability of cooling a wafer which is heated to a higher temperature. The electrostatic chuck having such a structure as set out in the above-indicated Japanese Laid-open Patent Application No. 59-64245 is insufficient to meet the above demand, i.e. the cooling capability of the chuck has been found to be unsatisfactory.